Combustion Method and System

ABSTRACT

A method of combustion for pulverized hydrocarbonaceous fuel includes the steps of injecting an air/fuel stream into a burner, causing a low-pressure zone; directing a flow of a high-temperature combustion gas from a combustion chamber into the low-pressure zone in the burner; mixing the high-temperature combustion gas with the injected air/fuel stream to heat the injected air/fuel stream, and injecting the heated air/fuel stream from the burner to the combustion chamber, wherein the air/fuel stream is rapidly devolatilized and combusted in a flame that has a high temperature; sensing a combustion parameter; and based on the sensed combustion parameter, controlling combustion to achieve at least one of a desired NOx reduction and a desired distance from the burner to a flame front.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a combustion method, and a combustionsystem, for solid hydrocarbonaceous fuel.

BACKGROUND OF THE INVENTION

Solid fossil fuel, such as coal, is an important energy source,particularly for power generation. Pollutants emitted from coalcombustion, however, are a major source of air pollution. Of thepollutants from coal combustion, nitrogen oxides (NOx) have attractedextensive attention.

There are two primary sources of NOx generated during combustion: fuelNOx and thermal NOx. Fuel NOx is NOx formed due to the conversion ofchemically bound nitrogen (fuel nitrogen) during combustion. Fuelnitrogen (or char-N) is released in several complex combustionprocesses. The primary initial product of combustion is either HCN orNH3. HCN is then either oxidized to NO or reduced to N₂. If the gasesare oxidant or the fuel is lean, NO will be the dominant product of fuelnitrogen. If it is fuel rich, HCN is reduced to N₂ by CO or C (char) onthe coal char surface.

Thermal NOx refers to NOx formed from high temperature oxidation ofatmospheric nitrogen. Thermal NOx formation is an exponential functionof temperature and a square root function of oxygen concentration. Alower combustion temperature or a lower oxygen concentration yieldslower NOx. Therefore, the production of thermal NOx can be controlled bycontrolling the reaction temperature or the oxygen concentration.However, a lower combustion temperature or a lower oxygen concentrationleads to an inefficient burning of coal, i.e., a slow burning rate. Aslow burning rate may result in an incomplete burning of coal and aprolonged burning of coal.

Various technologies have been developed to reduce NOx emission. Thesetechnologies either reduce the combustion temperature or manipulate theoxygen concentration. The first is called “dilution based combustioncontrol technique,” and the latter is referred to as “stoichiometrybased combustion control technique.” The dilution based combustiontechnique introduces inert gases such as water or flue gases to reducethe flame peak temperature. The stoichiometry based combustion techniqueinvolves lowering the oxygen concentration in the flame zone andgenerating a reducing atmosphere, thus allowing NOx to be reduced.Examples are low-NOx staged burners and OS combustion, e.g.,over-fire-air and burner-out-of-service. These techniques control NOxgeneration by providing air and/or fuel staging to create fuel-richzones (partial combustion zones) followed by air-rich zones to completethe combustion process. These low-NOx burners can reduce the NOxemission to 0.65 to 0.25 pounds per million BTUs. Another type of NOxcontrol technology is gas reburning. The reburning technology can lowerthe NOx emission to 0.45 to 0.18 pounds per million BTUs.

However, these NOx reduction techniques are less than adequate. Forexample, they cannot meet the emission requirements (less than 0.15pounds per million BTUs) under the U.S. Clean Air Act. Additionally, inalmost all low-NOx combustion techniques, the combustion time has to beincreased significantly. As a result, the boiler size must be increasedto accommodate the long combustion time so that coal combustion can becompleted at an economically acceptable level. Consequently, almost allthe NOx control technologies require significant capital investment, andthe cost of operation is high.

Recent studies have shown that feeding coal with high-temperature gascould significantly reduce NOx emission and unburned carbon in fly ash.In the combustion process with high-temperature gas, the fuel nitrogenis devolatilized rapidly, and reduced to nitrogen duringdevolatilization and combustion in a fuel rich zone.

SUMMARY OF THE INVENTION

The present invention is based on the inventors' recognition of severalproblems associated with the prior art. One of the problems is thatalthough the prior art technologies for reducing NOx are based on solidtheories, the devices based on the technologies often do not achieveoptimum NOx reduction. The reason is that those devices do not, orcannot quickly, adjust operating parameters to adapt to changingoperating conditions for optimum NOx reduction. For example, when thequality or type of coal changes or when the load is changed, the priorart devices do not, or cannot quickly, recognize the change and adjustthe operating parameters to adapt to the change. As a result, an optimumNOx reduction cannot be achieved for the coal being used. At the sametime, unburned carbon in fly ash also increases.

Another problem associated with the prior art is that, in the case ofthe technology involving feeding high-temperature gas to coal, whichproduces high combustion temperature, the failure to adjust operatingparameters to adapt to changing operating conditions may result in theflame front becoming too close to the wall of the burner and/or the wallof the combustion chamber. As a result, slagging takes place on the wallof the burner and/or the wall of the combustion chamber. For example,the inventors' experiment shows that when the operating parameters areset for anthracite coal (with volatile of 7.36%) but bituminous coal(with volatile of 17.22%) is used, slagging takes place on the wall ofthe burner due to over-heating and can cause a shout-down of thecombustion system.

The present invention is directed to a method of combustion that has oneor more advantages of low NOx emission, low unburned carbon, automaticadaptability to any types of fossil fuel, and reduced slagging. Thecombustion method may include injecting a air/fuel stream into a burnerto cause a low-pressure zone; directing a flow of a high-temperaturecombustion gas from a combustion chamber into the low-pressure zone inthe burner; mixing the high-temperature combustion gas with the injectedair/fuel stream to heat the injected air/fuel stream, and injecting theheated air/fuel stream from the burner to the combustion chamber,wherein the air/fuel stream is rapidly devolatilized and combusted in aflame; sensing a combustion parameter; and based on the sensedcombustion parameter, controlling the combustion to achieve at least oneof a desired NOx reduction and a desired distance from the burner to afront of the flame. In a preferred embodiment, the combustion iscontrolled to maximize NOx reduction without impermissible slagging.What constitutes “impermissible slagging” cannot be determined in theabstract and must be determined on a case-by-case basis from the designrequirements for a given combustion system. Such a determination can bemade by a person with ordinary skill in the art.

The present invention is directed also to a combustion system forpulverized hydrocarbonaceous fuel. A combustion system may include aburner that is designed to receive a air/fuel stream; a combustionchamber that is connected to the burner to send to the burner a flow ofa high-temperature combustion gas to heat the air/fuel stream, and toreceive the heated air/fuel stream form the burner for combustion; asensor for sensing a combustion parameter; and a controller forcontrolling the combustion based on the sensed combustion parameter toachieve at least one of a desired NOx reduction and a desired distancefrom the burner to a flame front. In a preferred embodiment, thecombustion is controlled to maximize NOx reduction without impermissibleslagging.

In a preferred embodiment, the velocity of the injected air/fuel streamin the burner is 10 to 60 m/sec, more preferably 15 to 50 m/sec. Thevelocity can be designed so as to feed the air/fuel stream withoutblocking the feed pipe, and to introduce a pressure inside the burnerthat is lower than that in the combustion chamber. The cross-sectionalarea of the injection at the entrance of the burner may be a fraction ofthe cross-sectional area of the burner, preferably 20% to 60%. Thedesirable ratio of the two cross-sectional areas allows a certain amountof high-temperature combustion gas to flow back into the burner from thecombustion chamber.

In another preferred embodiment, the air/fuel stream is a concentratedair/fuel stream, i.e., a air/fuel stream having a low air to fuel ratio.Preferably, the ratio of air to fuel solids in the concentrated streamis 0.4 to 2.2 kg air/1 kg fuel, more preferably 0.7 to 1.8 kg air/1 kgfuel. This represents only 8% to 25% of the stoichiometric ratio forfuels such as anthracite and bituminous coals.

There are several reasons for the use of a concentrated air/fuel stream.First, the concentrated stream allows the maintenance of a highlyfuel-rich flame inside the burner and combustion chambers, which cansignificantly reduce the NOx. Secondly, the concentrated stream can beheated up using a relatively small amount of heat. Thus the concentratedstream can be quickly heated up in a short distance. Third, the heatedconcentrated stream releases a large amount of volatiles in the fastheating. (Partial combustion also may take place during the heating ofthe concentrated stream.) The released volatiles enhance the ignitionand combustion of the coal particles, reducing the unburned carbon infly ash. Additionally, a fast release of volatiles including fuel-boundnitrogen in the fuel rich atmosphere allows transformation of thefuel-bound nitrogen into N₂ rather than NOx. The overall effects of theconcentrated air/fuel stream and the designed burner allow combustion tobe performed and maintained at a high temperature and in an atmosphereof reduced gases, which is conductible to ultra-low NOx emission and lowunburned carbon in fly ash.

The air/fuel stream in the burner can be a swirling flow or a straightflow. Some typical setups of the burner are wall fired, opposite fired,tangential fired, and down-fired. The burner preferably is arranged atthe same vertical elevation as that of the combustion chamber.

In still another preferred embodiment of the present invention, thecombustion system may include a separating device that is designed toseparate a air/fuel stream from a pulverizing system into theconcentrated air/fuel stream and a diluted air/fuel stream. Theseparating device is connected to the burner to supply the concentratedair/fuel stream to the burner. The ratio of air to fuel solids for theconcentrated stream is lower than that for the air/fuel stream from thepulverizing system. Typically, the ratio of air to the fuel solids inthe air/fuel stream from the pulverizing system may be 1.25 to 4.0 kgair/1 kg fuel. The ratio of air to fuel solids in the concentratedair/fuel stream preferably is 0.4 to 2.2 kg air/1 kg fuel, morepreferably 0.7 to 1.8 kg air/1 kg fuel.

In general, an embodiment of the present invention may include two ormore air/fuel streams that are injected into a combustion chamber. Eachof these air/fuel streams may be a concentrated air/fuel stream, whichmay have a ratio of air to fuel solids between 0.4 to 2.2 kg air/1 kgfuel, more preferably between 0.7 to 1.8 kg air/1 kg fuel.Alternatively, each of these air/fuel streams may be a diluted air/fuelstream, which may have a ratio of air to fuel that is greater than thatof a concentrated air/fuel stream. Each of the air/fuel streams may beheated, as described above, or unheated, before it is injected into thecombustion chamber.

For example, a preferred embodiment of the present invention may includea primary air/fuel stream that is concentrated and heated, and asecondary air/fuel stream that is diluted and may or may not be heated.Preferably, the primary air/fuel stream is first injected into thecombustion chamber, and then the secondary air/fuel stream is injectedinto the combustion chamber to complete the combustion. The secondaryair/fuel stream may contain sufficient oxygen that the total amount ofoxygen fed into the combustion chamber makes up at least thestoichiometric amount needed for a complete combustion of fuel.Preferably, the secondary air/fuel stream is fed into the combustionchamber adjacent to the exit of the burner for the primary stream. Atypical secondary air and fuel stream contains about 3.5 to 8.0 kg ofair for 1 kg of fuel, which represents about 65 to 90% of thestoichiometric combustion air required for a complete combustion ofanthracite coal, bituminous coal, and oil coke.

In this example, an additional diluted air/fuel stream, such as aso-called “over-fire air,” is injected into the combustion chamber. Thisadditional diluted air/fuel stream may or may not be heated. In someembodiments, the additional diluted air/fuel stream contains sufficientoxygen such that the total amount of oxygen fed into the combustionchamber is at least the stoichiometric amount for a complete combustionof fuel.

For another example, a preferred embodiment of the present invention mayinclude two or more concentrated air/fuel streams that may or may not beheated, and each of the concentrated air/fuel stream may be followed byone or more diluted air/fuel streams that may or may not be heated.

The controlling of combustion to optimize at least one of NOx reductionand the distance from the burner to a flame front may be carried out inseveral ways. For example, it may include controlling one or more of thefollowing control parameters: the pressure in the low-pressure zone in aburner, at least one of the flow rate and air/fuel ratio of aconcentrated air/fuel stream, and at least one of the flow rate andair/fuel ratio of a diluted air/fuel stream.

Combustion control can be achieved by controlling the pressure in thelow-pressure zone, because the pressure in the low-pressure zone affectsthe flow rate of the high-temperature combustion gas from the combustionchamber into the low-pressure zone in the burner and, thus, the heatingof the air/fuel stream. The pressure in the low-pressure zone can becontrolled by introducing a gas into the low pressure reflow zone.Preferably, the gas is air (tertiary air). When the quantity of tertiaryair is increased, the pressure in the low-pressure zone is alsoincreased, resulting in a decreased flow of the high-temperaturecombustion gas from the combustion chamber into the low-pressure zone.As a result, the heating of the air/fuel stream is reduced, andcombustion temperature may be reduced. The amount of tertiary airaffects also the air/fuel weight ratio of the air/fuel stream, which canalso be used for combustion control.

Combustion control may also be achieved by controlling the flow rate andair/fuel ratio of a air/fuel stream injected into the burner, becausethe flow rate and/or concentration of the air/fuel stream affect thepressure in the low-pressure zone and the devolatilization andcombustion of the air/fuel stream.

The combustion control of the present invention can be based on one ormore combustion parameters. Representative parameters may be combustiontemperature, pressure, and the concentration of one or more selectedgases such as carbon dioxide, carbon monoxide, oxygen and nitrogen.Preferably, the temperature is used as the combustion parameter. Thecontrol may be realized by sensing the value of the combustion parameterinside the burner and/or the combustion chamber, and comparing thesensed value with a preset value. Based on the difference between thesensed value and preset value, the controller, such as a close-loopcontroller or a distributed control system, adjusts one or more of theabove-discussed control parameters to reduce the difference. When thedifference is reduced, the NOx emission is reduced, and/or a desireddistance from the burner to a flame front is maintained to reduceslagging. This automatic control enables a burner to be used with almostall kinds of fuel without changing the structure of the combustionsystem.

Herein, the term “reflow” means a flow of the high-temperaturecombustion gases from the combustion chamber back to the burner. Theflow of the combustion gases is in the opposite direction of the fuelstream. Other terms for such types of flow are “reflux” and“recirculation.” The reflow is caused by the pressure reduction resultedfrom the injection of the air/fuel stream into the burner.

Herein, the term “heating” means heating of the air/fuel stream in theburner. The heating source is from the reflow of the high-temperaturecombustion gases. The heating may be conducted by mixing and thermalradiation. In the case of the concentrated air/fuel stream, thetemperature of the air/fuel stream may reach 700° C. to 1200° C. in adistance ranging between 250 mm and 1950 mm measured from the exit ofthe feeding pipe for the concentrated fuel stream to the burner.

Herein, the term “NOx” means oxides of nitrogen, including NO, NO₂, NO₃,N₂O, N₂O₃, N₂O₄, N₃O₄, and their mixtures.

Herein, the term “bound nitrogen” means nitrogen that is a compositionof a molecule that composes of carbon and hydrogen and possibly oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a preferred embodiment of the inventionfor creating a concentrated fuel stream and performing heating in theburner and combustion in a combustion chamber.

FIG. 2 shows the flow pattern for reflow and heating of the air/fuelstream.

FIGS. 3 and 4 show cross section of a burner of the embodiment shown inFIG. 1

FIGS. 5 and 6 show cross-sectional representations of devices used inthe present invention for feeding a concentrated fuel stream to thecombustion chamber, for creating reflow of high-temperature combustiongases back into the burner, and for controlling the re-flow ofhigh-temperature combustion gases back into the burner.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preferred embodiments of the present invention described below arediscussed sometimes in terms of coal combustion, and in terms of airbeing the gaseous carrier and oxidant. The techniques described areapplicable to any other pulverized solid fuel and any other gaseouscarrier. The invention will be described with the aid of the Figures,yet a description that refers to the Figures is not used to limit thescope of the invention.

FIG. 1 to 4 show a preferred embodiment of a swirling burner accordingto the present invention. Some embodiments of the burner are describedin more detail in FIGS. 4 and 5. The invention also encompassesstraight-flow burners where the secondary stream or/and the otherstreams is (are) fed into the combustion chamber in a straight flow.

FIG. 1 shows a combustion system includes a burner 3 and a combustiondevice 1 having a chamber 2. The combustion device of the presentinvention can be any apparatus within which combustion takes place.Typical combustion devices include furnaces and boilers. A burner 3 ismounted on a sidewall or at a wall corner of the combustion device 1 andfeeds fuel solids and air from sources outside the combustion device 1into the combustion chamber 2 of the combustion device 1. Typical fuelsinclude pulverized hydrocarbon solids, an example of which is pulverizedcoal or petroleum coke.

In the illustrated embodiment, fuel and air are supplied to thecombustion system as a main air/fuel stream A, and a secondary dilutedair/fuel stream for an aerodynamic control of the mixing between thefuel and the air. In the main air/fuel stream A, the air may be suppliedwith a stoichiometric ratio less than 1. The air used to complete thecombustion of the fuel may be supplied to the combustion device 1 as thesecondary stream B (=B₁+B₂) and/or as an over-fire air as shown in FIGS.1 to 4.

As shown in FIGS. 1 and 3 to 6, the burner 3 is comprised of an injector8, 16 for a primary concentrated air/fuel stream a₁, a secondary streaminjector 13, 19, and an automatic control unit 30. Preferably, asolid-gas separator 4 is placed in front of the injector 8 for theprimary concentrated air/fuel stream a₁ to separate the main air/fuelstream A into a concentrated stream a₁ and a diluted fuel stream a₂. Theseparator 4 is preferred to be a bent three-way separator but should notbe limited to a bend separator. The bent three-way separator 4 includesa main-stream inlet pipe 5, a bent pipe 6, a feeding pipe 7 for adiluted stream a₂, and a feeding pipe 8 for the primary concentratedfuel stream a₁. Preferably, the winding angle of the bent pipe 6 isbetween 60° and 120°. The ratio of the inner radius of the pipe 8 forthe concentrated air/fuel stream to the inner radius of the pipe 7 forthe diluted fuel stream is between 0.5 and 2.0.

The main air/fuel stream A from a pulverizing system (not shown in thefigure) may be fed from the inlet pipe 5 through the bent 3-wayseparator 4 at a velocity. Fuel powders can be concentrated on the outerbend of the separator 4 by the design of the separator 4 with aspecified radius and a winding angle to match the flow velocity. Thisseparates the main stream A into the primary concentrated stream a₁ inthe outer region of the bend and a diluted stream a₂ in the inner regionof the bend. The concentrated stream a₁ is fed to the burner 3 through afeeding pipe 8. Through a feeding pipe 7, the diluted stream a₂ is fedthrough a port 20 into the combustion device 1 at a location close tothe burner 3. The angle in the exit direction of the separator 4 can beadjusted. A typical main stream A contains about 1.25 to 4.0 kg of airfor 1 kg of fuel solids, which represents about 10 to 35% of thestoichiometric combustion air required for a complete combustion of thefuel.

The flow rate and concentration of the concentrated stream a₁ or dilutedstream a₂ can be controlled by adjusting a flap valve 27 disposedbetween the feeding pipe 8 for the concentrated stream a₂ and thefeeding pipe 7 for the diluted stream a₂. Alternatively, some otherarrangement may be made to control the flow rate and concentration ofthe concentrated stream a₁ or diluted stream a₂.

The secondary stream is from the secondary stream windbox 11 (FIG. 1).Preferably, the secondary stream is fed using two passages: an innersecondary stream passage B₁ and an outer secondary stream passage B₂.The inner secondary stream passage B₁ includes a throttle 9 for thestraight-flow secondary stream, a throttle 10 for the swirling-flowsecondary stream, an air deflector 12, and a secondary stream spurt pipe13. The outer secondary stream passage B₂ includes a throttle 14 for thestraight-flow secondary stream, a throttle 15 for the swirling-flowsecondary stream, an air deflector 18, and a secondary stream spurt pipe19. Those components are placed concentrically along the axis of the fedline 16 of the concentrated stream a₁ if the components are in acircular or cylindrical shape.

Fed from the windbox 11, the inner secondary stream B₁ is then separatedinto two streams by adjusting the throttles 9 and 10. Of them, the firststream b₁₁ is a straight-flow air, the second stream b₁₂ is a swirlingflow air produced by the axial air deflector 12. Adjusting the throttles9 and 10 allows a desirable swirling strength. Fed from the windbox 11,the outer secondary stream B₂ is then separated into two streams byadjusting throttles 14 and 15. Of them, the first stream b₂₁ is astraight-flow air, the second stream b₂₂ is a swirling flow produced bythe axial air deflector 18. Adjusting the throttles 14 and 15 allows adesirable swirling strength. A typical secondary stream B contains about3.5 to 8.0 kg of air for 1 kg of fuel, which represents about 65 to 90%of the stoichiometric combustion air required for a complete combustionof anthracite, bituminous coals and oil coke. The swirl strength iscontrolled by adjusting throttles 9 and 10 and 14 and 15. Preferably, aswirl number, as defined in “Combustion Aerodynamics”, J. M. Beer and N.A. Chigier, Robert E. Krieger Publishing Company, Inc., 1983, is 0.1 to2.0.

Preferably, an over-fire air is fed through an over-fire-air port 21into the combustion device 1 to make the entire combustion zone insidethe combustion device 1 fuel-rich and supplies more oxygen to help acomplete combustion of the fuel. The volume percentage of theover-fire-air may be between 0 and 30% of the total air sent to thecombustion device 1 that is required for a complete combustion of thefuel.

In a preferred embodiment, the concentrated stream enters the burnerchamber 40 and forms a fuel-rich zone C₁ where the stoichiometric ratiois between 0.08 and 0.25. A reflow of high-temperature gas is introducedinto the burner 3 from the combustion chamber 2 to heat rapidly theconcentrated stream to devolatilize volatiles and bound nitrogen. Andcombustion takes place between the fuel solids and the combustion airsequentially, producing a flame C₂. The secondary stream and sometimesthe over-fire air are injected into the combustion chamber 2 to completecombustion. The reflow is caused by the relatively lower pressure causedby the injection of the concentrated stream a₁ at a relatively highvelocity compared to the velocity of gases inside the combustion device1.

The rapid heating of the concentrated fuel stream in the fuel-rich zoneC₁ generates a volatile fuel-rich zone. This significantly increases thecombustibility of the fuel stream. Thus ignition is maintained andcompleted in a short time and range. And fuel combustion can bemaintained at a high temperature. Rapid heating and devolatilizationcombined with high-temperature combustion under an atmosphere ofreducing gases generate nitrogen. These exactly same combustionconditions also help the combustion of fuel particles and thus reducethe unburned carbon in the fly ash.

When the fuel concentration is higher or the ratio of air/fuel issmaller, the ignition time will be shorter; the combustion temperaturewill be higher; and the flame front is closer to the burner. When theflame front is too close to the mouth of the burner, for example,slagging may occur. This is especially important when the fuel typechanges from a low grade fuel with a low content of volatiles such asanthracite coal to a fuel with a high content of volatiles such as thebituminous coal. In this case, the ratio of air/fuel should be increasedto prevent slagging.

The invention uses a sensor 22 to monitor the change of at least oneparameter in the burner 3 or in the combustion chamber 2. Representativeparameters include temperature, pressure, and the content of a selectedgas. The selected gas can be one or more of O₂, CO, CO₂, NOx, N₂, andHC. The sensor can be placed in the burner 3 or in the combustionchamber 2, or in an area where the burner 3 and the combustion device 1intersect. For example, the temperature sensor may be placed at or neara location where slagging is likely to take place. The temperaturesignal is sent to a closed-loop controller 23.

A typical controllers may be a PID (proportional-integral-differential)controller or a DCS (distributed control system) controller. The signalis compared to a pre-set value. If the detected temperature signal islarger than the pre-set value, meaning that the combustion temperatureis too high or that the flame front is closer than the desired distancefrom the burner, the controller sends a command to the servo-motor 24,which then varies the opening of the valve 25 to reduce combustiontemperature. Specifically, the controller may allow more tertiary air T(directly from the atmosphere or from a supplying source) into theburner 3. The additional tertiary air dilutes the fuel stream andreduces combustion gas reflow, increasing the distance between theburner 3 and the flame front. The control process automaticallycontinues until the sensed temperature is the same or sufficiently closeto the desired value. The automatic control allows the combustion systemto be adaptable to different types of fuel and to reduce NOx emissions.

Preferably, the total amount of air fed to the combustion device 1,i.e., the sum of the air in the main air A (=a₁+a₂), the secondarystream B (=B₁+B₂), and the tertiary air T, is between 90 to 125% of thestoichiometric air required for complete the combustion. Preferably, theair through the over-fire-air port 21 is about 0 to 30% of the total airsent to the combustion device 1. The amount of over-fire air can becontrolled by adjusting the opening of the over-fire air valve 26.

Preferably, the tertiary air T is controlled such that the flame frontis at a location between 100 mm and 1400 mm from the burner. In somecases, when the flame front is closer to the burner than this preferredrange, slagging tends to occur.

The amount of air fed to the burner 3 and the arrangement of theaerodynamics of the air preferably is used to establish a stoichiometricratio in the fuel-rich zone of the flame C₂ that is less than 0.75. Theamount of air in the concentrated stream a₁ is preferably less than 30%of the stoichiometric amount required for the complete combustion of thesolid fuel. More preferably, the amount should be less than 20% of thestoichiometric amount.

Both the NOx emission and the unburned carbon in the ash depend on thestoichiometric ratio in the fuel-rich zone C₁ and the fuel-rich flamezone C₂ and on the heating rate or the temperature rising rate of thefuel-rich zone C₁. For example, if the main stream A is directly sent tothe burner 3, the heat required to heat the stream to the ignitiontemperature is about or more than two times of that required to heat theconcentrated stream a₁. As a result, the ignition of the fuel streamwill be delayed, and the combustion may not be completed in thecombustion system. At the same time, NOx emission is increaseddramatically when the stoichiometric ratio is larger than 1.0.

In a preferred embodiment, the present invention creates and maintains acontrolled fuel rich flame by: concentrating the conventional primarystream; then fast heating the concentrated stream using reflowedcombustion gases inside the burn 3 (the reflow is caused by the negativepressure induced by the relatively high-speed concentrated fuel streamitself); and controlling the reflow using a control system. The flame ofthe highly concentrated fuel stream is preferably maintained by thecontrolled reflow, allowing a stoichiometric ratio well below theoriginal primary air values.

Fuel injectors in burners generally have a circular cross section, anannual cross section (formed by two concentric pipes), or a square orrectangular cross-section (for example, injectors in tangentially firedboiler). These designs or layouts fulfill two functions for the presentinvention: feeding fuel streams into the combustion device, andgenerating the reflow of high-temperature gases back into the burnerthat is used to heat the concentrated stream. FIGS. 5 and 6 show somerepresentative designs that perform such functions. The presentinvention, nonetheless, includes all designs or layouts that feed thefuel and generate re-flow of high-temperature gases from the combustiondevice 1. These designs can be used in wall-fired boilers, thetangentially fired boiler, and the down-fired boilers.

FIG. 5 shows some fuel injectors that are without a tertiary air inlet.It should be pointed out that while some embodiments of the presentinvention use the tertiary air to control the pressure in the lowpressure reflow zone, other embodiments of the present invention alsoinclude a burner that does not use the tertiary air. In FIG. 5 a, thefeeding pipe 8 for a concentrated fuel stream is at the centerline of aburner pipe 16. In FIG. 5 b, the feeding pipe 8 is located off thecenterline of the burner pipe 16. In FIG. 5 c, the feeding pipe 8 isarranged around the burner pipe 16. In FIG. 5 d to 5 g, the feeding pipe8 is composed of two parts: a straight section and a concentric section,and inside the burner pipe 16, there could include a solid. When thetertiary air is not used to control the pressure of the low-pressurezone in the burner 3, the amount and/or content of the concentrated fuelstream flowing into the burner may be controlled to adjust the pressureinside the burner and/or to adjust the heating and the weight ratio offuel/air in the burner 3.

FIG. 6 shows some fuel injectors that have a tertiary air inlet. In FIG.6 a, the tertiary air inlet is located on a side wall of the burner pipe16. Preferably, a tertiary-air pipe 17 is located in the first twothirds of the burner pipe 16 (from the fuel-stream entrance). In FIG. 6b, the tertiary air inlet 17 is located on the front surface (herein thefront is the entrance of the fuel stream) of the burner pipe 16.

The burner pipe 16 and the tertiary-air pipe 17 can be of any shape.Representative shapes are cylindrical, cubic, prismatic, cone-shaped,elliptic, and frustum-shaped of pyramid. Additionally, all feeding pipes8 and burner pipes 16 shown in FIG. 5 can be used as fuel injector withtertiary air. The preferable shapes are cylindrical, cuboid, andprismatic. There can be any number of feeding pipes for the concentratedfuel stream and tertiary-air pipes. The tertiary pipe 17 can be at anyangle with respect to the burner centerline.

1-48. (canceled)
 49. A method of combustion for pulverizedhydrocarbonaceous fuel, the method comprising: injecting an air/fuelstream into a burner, causing a low-pressure zone; directing a flow of ahigh-temperature combustion gas from a combustion chamber into thelow-pressure zone in the burner; mixing the high-temperature combustiongas with the injected air/fuel stream to heat the injected air/fuelstream, and injecting the heated air/fuel stream from the burner to thecombustion chamber, wherein the air/fuel stream is rapidly devolatilizedand combusted in a flame that has a high temperature; sensing acombustion parameter; and based on the sensed combustion parameter,controlling combustion to achieve at least one of a desired NOxreduction and a desired distance from the burner to a flame front.
 50. Amethod according to claim 49, wherein the step of controlling thecombustion includes controlling the pressure of the low-pressure zone.51. A method according to claim 50, therein the step of controlling thepressure of the low-pressure zone includes controlling a tertiary airfed into the low pressure zone to control the pressure of thelow-pressure zone.
 52. A method according to claim 51, wherein a feedingpipe for feeding the tertiary air is located in the first two-third ofthe burner measured from its entrance for the air/fuel stream.
 53. Amethod according to claim 49, wherein the step of controlling thecombustion includes controlling the flow rate of the high-temperaturecombustion gas from the combustion chamber into the low-pressure zone inthe burner.
 54. A method according to claim 49, wherein the step ofcontrolling the combustion includes controlling at least one of the flowrate and air/fuel ratio of the injected air/fuel stream.
 55. A methodaccording to claim 49, wherein the air/fuel stream is a concentratedair/fuel stream.
 56. A method according to claim 55, wherein theconcentrated stream has a weight ratio of air to fuel in the range of0.4 to 2.2.
 57. A method according to claim 55, wherein the concentratedstream is heated to a temperature of 700° C. to 1200° C. in a distancebetween 250 mm and 1950 mm as measured from the entrance of the burnerfor the high-temperature gas.
 58. A method according to claim 55,wherein the concentrated stream has a weight ratio of air to fuel in therange of 0.7 to 1.8.
 59. A method according to claim 55, wherein theconcentrated stream is injected into the burner at a speed from 10 to 60m/s.
 60. A method according to claim 55, wherein the concentrated streamis injected into the burner at a speed from 15 to 50 m/s.
 61. A methodaccording to claim 49, wherein a cross-sectional area of the injectedair/fuel stream at the entrance to the burner is a fraction of across-sectional area of the burner.
 62. A method according to claim 61,wherein the cross-sectional area of the injected air/fuel stream at theentrance to the burner is less than 50% of the cross-sectional area ofthe burner.
 63. A method according to claim 49, wherein the fuel is atleast one of coal and oil coke.
 64. A method according to claim 55,further comprising separating a primary air/fuel stream into theconcentrated air/fuel stream and a diluted air/fuel stream, and feedingthe diluted stream into the combustion chamber.
 65. A method accordingto claim 64, wherein the step of controlling the combustion includescontrolling the feeding of the diluted stream into the combustionchamber.
 66. A method according to claim 64, wherein the separating ofthe primary air/fuel stream into the concentrated stream and the dilutedstream is performed by a bent pipe.
 67. A method according to claim 66,wherein the winding angle of the bent pipe is between 60° and 120°. 68.A method according to claim 64, wherein the primary stream contains 10%to 35% of stoichiometric air.
 69. A method according to claim 49,wherein the combustion parameter includes at least one of a pressuresensor, a temperature sensor, and a chemical sensor for sensing thecontent of a gas.
 70. A method according to claim 49, wherein thesensing step is performed by a sensor that is placed in the burner orcombustion chamber or embedded in a wall of the burner or combustionchamber.
 71. A method according to claim 49, further comprisinginjecting at least one additional air and fuel stream.
 72. A methodaccording to claim 71, further comprising heating one of the at leastone additional air and fuel stream by an additional reflow of ahigh-temperature combustion gas from the combustion chamber.
 73. Amethod according to claim 71, wherein one of the at least one additionalair and fuel stream is an over-fire air, wherein the over-fire air is 0to 30% of the total air fed to the combustion chamber.
 74. A methodaccording to claim 73, wherein the step of controlling the combustionincludes controlling the feeding of the over-fire air.
 75. A methodaccording to claim 71, wherein one of the at least one additional airand fuel stream is a secondary diluted air and fuel stream.
 76. A methodaccording to claim 75, further comprising feeding the secondary streamto the combustion chamber adjacent to the periphery of the exit of theburner for the first air/fuel stream.
 77. A method according to claim75, wherein the step of controlling the combustion includes controllingthe feeding of the secondary stream.
 78. A method according to claim 75,wherein the secondary stream is one of a straight flow or a swirlingflow.
 79. A method according to claim 78, further comprising dividingthe swirling secondary stream into an inner secondary stream and anouter secondary stream.
 80. A method according to claim 79, wherein theswirling strength is between 0.1 and 2.0.
 81. A method according toclaim 71, wherein the first air/fuel stream is a first concentratedair/fuel stream, and wherein one of the at least one additional air andfuel stream is a second concentrated air and fuel stream.
 82. A methodaccording to claim 81, wherein the second concentrated air/fuel streamis heated.
 83. A method according to claim 49, wherein the step ofcontrolling combustion includes controlling combustion to maximize NOxreduction without impermissible slagging.
 84. A combustion system forpulverized hydrocarbonaceous fuel, the device comprising: a burner thatis designed to receive an air/fuel stream; a combustion chamber that isconnected to the burner to send to the burner a flow of ahigh-temperature combustion gas to heat the air/fuel stream, and toreceive the heated air/fuel stream from the burner for combustion; asensor for sensing a combustion parameter; and a controller forcontrolling combustion based on the sensed combustion parameter toachieve at least one of a desired NOx reduction and a desired distancefrom the burner to a flame front.
 85. A system according to claim 84,wherein the controller controls the pressure of the low-pressure zone.86. A system according to claim 85, therein the controller controls atertiary air fed into the low pressure zone to control the pressure ofthe low-pressure zone.
 87. A system according to claim 84, wherein thecontroller controls the flow rate of the high-temperature combustion gasfrom the combustion chamber into the low-pressure zone in the burner.88. A system according to claim 84, wherein the controller controls atleast one of the flow rate and air/fuel ratio of the injected air/fuelstream.
 89. A system according to claim 84, wherein the air/fuel streamis a concentrated air/fuel stream.
 90. A system according to claim 84,wherein the combustion parameter includes at least one of a pressuresensor, a temperature sensor, and a chemical sensor for sensing thecontent of a gas.
 91. A system according to claim 84, wherein at leastone additional air and fuel stream is injected into the combustionchamber.
 92. A system according to claim 91, wherein one of the at leastone additional air and fuel stream is heated by an additional reflow ofa high-temperature combustion gas from the combustion chamber.
 93. Asystem according to claim 91, wherein one of the at least one additionalair and fuel stream is a secondary diluted air and fuel stream.
 94. Asystem according to claim 91, wherein the first air/fuel stream is afirst concentrated air/fuel stream, and wherein one of the at least oneadditional air and fuel stream is a second concentrated air and fuelstream.
 95. A system according to claim 94, wherein the secondconcentrated air/fuel stream is heated.
 96. A system according to claim84, wherein the controller controls combustion to maximize NOx reductionwithout impermissible slagging.